Precursor and effector CTL specific for determinants present on allogeneic MHC class I molecules have previously been shown to bear the Lyt2 cell surface phenotypic marker. Lyt2 is believed to be involved in the function of these cells because antibodies directed against Lyt2 block the cytolytic activity of most in vitro-generated MHC class I allospecific CTL. Based on those studies, Lyt2 has been proposed to be either an accessory molecule contributing to antigen specificity, or an "off" signal. Our studies have focused on a novel population of class I-specific CTL, which are induced in vitro in the presence of anti-Lyt2 monoclonal antibody and are refractory to functional blocking by subsequent exposure to anty-Lyt2 antibody. These novel CTL are Lyt2+ and adsorb surface anti-Lyt2 monoclonal antibody during culture. However, unlike conventional Lyt2+ CTL, they exhibit selective resistance to lysis with anti-Lyt2 antibody and complement. Since CTL with this unusual characteristic are generated only in the presence of anti-Lyt2 antibody, Lyt2 may serve as a positive signal transducing molecule on the surface of some T cells. Thus, Lyt2 may act as on "on" signal during CTL differentiation.